1. Field of the Invention
This invention relates generally to cardiac stimulator leads, and more particularly to a cardiac stimulator lead having a two-piece connector for connecting the lead to cardiac stimulator, such as a pacemaker or cardioverter/defibrillator.
2. Description of the Related Art
Conventional cardiac stimulator systems consist of a cardiac stimulator and an elongated flexible cardiac lead that is connected proximally to a header structure on the cardiac stimulator and is implanted distally at one or more sites within the heart requiring cardiac stimulation or sensing. The cardiac stimulator is normally a pacemaker, a cardioverter/defibrillator, a sensing instrument, or some combination of these devices.
At the time of implantation, the distal end of a cardiac lead is inserted through an incision in the chest and manipulated by the physician to the site requiring electrical stimulation with the aid of a flexible stylet that is removed prior to closure. At the site requiring electrical stimulation, the distal end of the lead is anchored to the endocardium by an active mechanism, such as a screw-in electrode tip, or alternatively, by a passive mechanism, such as one or more radially spaced tines that engage the endocardium. The proximal end of the lead is then connected to the cardiac stimulator and the incision is closed. The implantation route and site are usually imaged in real time by fluoroscopy to confirm proper manipulation and placement of the lead.
A conventional cardiac stimulator lead normally consists of an elongated flexible tubular, electrically insulating sleeve that is connected proximally to a connector that is adapted to couple to the header of a cardiac stimulator, and distally to a tubular tip electrode. One or more ring-type electrodes may be secured to the sleeve at various positions along the length of the sleeve. The proximal end of the lead sleeve is connected to the connector by application of various biocompatible adhesives to various portions of the connector and the sleeve. The tip electrode ordinarily consists of a tubular structure that has an increased diameter portion that forms an annular shoulder against which the distal end of the lead sleeve is abutted. The exterior surface of the tubular structure is normally smooth as is the interior surface of the distal end of the lead sleeve.
In many conventional cardiac stimulator leads, engagement between the one or more conductor wires of the lead and the connector is established by compressing the proximal ends of the wires between two concentrically positioned tubular structures. The compression is normally accomplished by crimping the outer of the two structures inward to compress the wire against the outer surface of the inner tubular structure. Establishment of the crimped connection involves aligning the cooperating tubular structures concentrically and longitudinally, feeding a sufficient portion of the conductor wire between the crimping tubular members, and crimping the tubular members to establish the crimped engagement.
There are several disadvantages associated with conventional lead connectors. Successful crimping of the cooperating tubular structures in the conventional connector requires special attention to be paid to dimensional tolerances and to the hardness of the materials used to fabricate the tubular structures. The load applied to crimp the tubular structures is based upon an expected hardness of the material as well as the design tolerance between the cooperating members. In circumstances where the tolerances between the cooperating tubular structures do not match design tolerances and/or the hardness of the tubular structures is greater than expected, the load applied may not be sufficient to successfully crimp and secure the conductor wire to the connector. As a result, the wire may make no or only intermittent electrical contact with the connector.
Verification testing of a conventional connector usually cannot identify a malfunction due to unsatisfactory crimping until the entire connector is fully assembled. This stems from the fact that the crimping step is one of the last steps in the assembly of a conventional connector. Since the crimping operation is not readily reversible, the ordinary remedy is to discard the malfunctioning connector. Aside from the material cost, another by-product of the inability to conduct interim verification test and/or visual inspection of the crimped connection is the costly expenditure of labor on a connector that is ultimately discarded.
Although a combination of crimping and adhesives is commonly employed to secure the proximal ends of a lead sleeve and the conductor wire(s) to the connector, the connection between the distal end of the lead sleeve and the tip electrode for most conventional cardiac leads is accomplished by use of an adhesive alone. A biocompatible adhesive, such as silicone based adhesive, is applied to the exterior of the tubular structure and the distal end of the lead sleeve is slipped over the tubular structure.
Many conventional lead designs incorporate a tip electrode that is composed of a non-radiopaque material. Although the motivations for selecting a non-radiopaque material for the tip electrode are several, a principle reason for selecting such materials is their ability to resist corrosion and maintain a relatively constant threshold voltage during long term exposure to the relatively hostile endocardial environment.
In addition to the aforementioned difficulties associated with conventional lead connectors, there are several other disadvantages associated with conventional designs for cardiac leads, and particularly the structure of the interface between the lead sleeve and the tip electrode. As noted above, a biocompatible adhesive is used as the dominant mechanism for securing the distal end of a lead sleeve to a tip electrode. To ensure that an adequate bond is formed between the adhesive and the mating surfaces of the lead sleeve and the tip electrode, most adhesives must be allowed to cure for durations of up to eight hours or more. This represents a significant bottle neck in the manufacturing and assembly process since the partially assembled lead must be set aside without further handling while the adhesive is allowed to cure.
Aside from manufacturing disadvantages, adhesives used for the sleeve-to-electrode joints may experience decreases in bond strength over time. The decrease may be caused by reactions with body fluids or tissues or may stem from inconsistent mixing and/or chemical makeup at the time of assembly. As a result, there exists a small risk that the lead sleeve may disconnect from the tip electrode in circumstances where an axial force is applied to the proximal end of the lead sleeve, such as when the lead is removed from the patient.
A lack of radiopacity is a shortcoming associated with conventional electrode tips that are composed of non-radiopaque material. Proper positioning of such leads is often a difficult task since the tips of such leads are not readily visible via fluoroscopy. In such circumstances, physicians often rely on the radiopaque character of the conducting coils inside the lead as an indicator of the position of the lead tip. However, for more modern leads incorporating individual small gage conductor wires, reliance upon the fluoroscopic visibility of the conductor wire may be insufficient as such fine wires normally do not show up clearly during fluoroscopy. A possible solution to the problem involves the incorporation of one or more radiographic markers into the lead sleeve. This technique involves additional expense and potentially complex manufacturing processes.
The present invention is directed to overcoming or reducing one or more of the foregoing disadvantages.